[I]t's a stretch to say that the "old time political bosses" had the power to dump nominees who became embarrassments. Yes, party bosses had a great deal of power to hand pick candidates and secure their nomination by controlling nominating conventions. But once a nominee had been selected, it was no small thing to remove them, which is why selecting the right nominee has always been such a crucial task. An Akin situation 150 years ago likely would have had similar results. It's not like some party boss would have kidnapped or murdered him. (I very much doubt that the kind of political violence portrayed in "Gangs of New York" was ever widespread in the United States, even in mid-1800s New York City.)I don't disagree, but there are some procedural issues here that are worth thinking about. 150 years ago is before the Australian, state-printed ballot, as well as before direct primaries. During that era, the parties supplied ballots to voters, who just turned them in at the polls; you weren't getting a ballot from election officials and then marking which candidates to vote for. Nominations weren't officially registered and adjudicated by the government; they were purely internal choices of private organizations. But that meant that a nomination, once obtained, didn't really belong to a nominee in the way that it does now; there was no official, formally state-certified, nominee. The real limiting factor (and I'm basically just following Alan Ware's The American Direct Primary in all of this) seems to have been the actual printing of the ballots -- remember, by the party, not by the government. And even then if a local party group didn't like the nominee they could replace him by "knifing" him off the ballot or replacing him with a "paster" stuck on the ballot instead of the regular nominee. So presumably up to the point of printing the ballots it would be easy for the party's leaders to substitute a new candidate if everyone wanted to dump the old one, while after the ballots were printed it would be a much more complicated job, but certainly possible. The main point here is that as far as I know the nominee up until the Australian ballot was introduced had no government-sanctioned right to a ballot line at all.
What really changes things, I think, is giving the nominee enforceable legal rights to a ballot line, which is a consequence of the Australian ballot, really -- indeed, the direct primary is partially a consequence of the Australian ballot, because once you have a legal ballot line that you're fighting for then you need procedures that will get you an unambiguous nominee, and a primary election turned out to be a good solution to that. After that, what probably matters is the specific way that the (state) laws are written; it's certainly possible to write a law that allows state party committees to overturn the results of a primary whether the nominee likes it or not, but as far as I know such laws are rare or non-existent. I assume, however, that there is at least some variation across states on the key question of whether the line belongs to the nominee or to the party, but you would need a real election law expert to help with that one.
The rest of what Seth says I agree with completely. And even if parties retain complete legal authority, it's still likely that a nominee might have considerable political leverage. For example, in the 19th century scenarios it's possible that a party might dump Akin completely after nomination -- but if he wanted to fight on, at the very least any personal loyalists he had could if they had the money create their own ballots with Akin on them and attempt to circulate them. Even worse for the party, if a significant party faction -- in this case, the pro-lifers -- stuck with him after the party dumped him, they could certainly keep him in play by printing their own ballots or altering the party-supplied ones; the result would almost certainly mean losing the election (and could be worse -- the pro-Akin faction could threaten, for example, to circulate "Republican" ballots with Akin but without Mitt Romney, or perhaps without some other GOP candidates).
Indeed, that kind of chaos was exactly why strong party leaders supported first the Australian ballot and then direct primaries. The combination meant that the party only had to arrange for it's candidate to win the official nomination; it no longer would have to enforce that nomination in every county and town. That was a big win for the parties...as long as they could control the nomination. Unfortunately for them, primaries turned out to be sometimes very difficult to control, although I don't think we have clear evidence of exactly how important that was in the demise of the old 19th century version of strong parties.
But as for dumping candidates: 150 years ago, it would have been easier as far as procedures and the law were concerned -- but a candidate would still have had considerable leverage, at least if he retained support of a significant faction of the party or had other resources which would have allowed him to threaten to go on alone. The big difference, however, is that the lack of a government-certified ballot position would mean he would have lacked one significant resource in the fight against his party.